The present invention relates to an optical system for testing infrared or ultraviolet sensors.
An increasing number of optical sensors use spectral regions outside that of visible light. Examples include warning sensors which are intended to discover incoming rockets by using the optical signature in the ultraviolet or in the infrared spectrum. Such sensors register a large spatial angle range and, if a risk is detected, are also intended to specify the direction of the threat in order that countermeasures can be initiated specifically.
In order to test such sensors, an optical test signal is generated in the appropriate spectral region and projected into the aperture of the sensor. Here, precise alignment of the projection axis may be required, for example in order to determine the resolution and accuracy of the directional information output or in order to direct a highly focused test signal precisely onto the sensor aperture even from a relatively great distance. The latter is the case when a sensor in the installed state outside the laboratory must be stimulated from a relatively large distance for test purposes because of restricted accessibility.
According to related techniques, axial alignment of a test projector is achieved by a “two-eyed” optical arrangement, in which a projector and a telescope are constructed separately from each other. Commercial systems for sensor stimulation are offered under the designation “Barringas”, for example by companies like CI Systems or Polytec.
Before the use of such a test projector, it is necessary first to align the two optical axes of projector and telescope parallel to each other. To this end, as a rule the telescope is aligned with a sensor set up at a suitable distance, and the optical axis of the projector is adjusted to maximize the sensor signal. The accuracy of the axial alignment that can be achieved in this way is limited, and is inadequate for a test of the directional resolution of a warning sensor.
A higher accuracy can be achieved during the adjustment of the optical axes by means of autocollimation with the aid of a suitable test reflector. However, this requires the replacement of the radiation source in the projector by a suitable detector which is sensitive in the spectral region of the projector. Such detectors are generally very expensive, in particular if a cooled IR detector is necessary. For this method, the accuracy of the alignment is limited by the accuracy of the test reflector, since it is struck by the two beam paths in two different regions, the reflective surfaces of which beam paths must ideally be parallel to each other.
One object of the present invention, therefore, is to provide an arrangement which is compact and easy to handle, and with which it is possible to test the directional resolution of an IR or UV sensor and to stimulate such a sensor with a test signal with optimal intensity, even from a great distance.
This and other objects and advantages are achieved by the optical system according to the invention, which comprises input optics, output optics having a reticule disposed on the associated optical axis, a radiation source which emits radiation intensity in the visual spectral region and in the infrared or ultraviolet spectral region, and a beam splitter for simultaneously visualizing an object scene illuminated by the radiation source with the reticule through the output optics, into the eye of an observer. The input optics comprise a lens, the imaging properties of which in the visual spectral region are equal to its imaging properties in the infrared or ultraviolet spectral region, and the beam splitter is a dichroic beam splitter.
The input optics are divided by the beam splitter into two separate beam paths having two image planes. In the image plane of one beam path (telescope), the image of an observed object scene in the visual spectral region coincides with the reticule disposed in this image plane to mark the optical axis, and is presented to an observer through the output optics for consideration. In the beam path of the other image plane (projector), the radiation source is disposed on the optical axis.
Therefore, the optical arrangement combines the beam paths of the projector and of the telescope via the beam splitter in such a way that the input optics, which are advantageously formed as a bi-spectral lens, are used jointly.
The bi-spectral lens of the input optics is expediently formed as an achromat, the chromatic correction state of which minimizes the secondary spectrum in the two spectral regions used. If, in addition to the visual region for the optical axial alignment, the IR region is used for the projection of the test signal, this correction state is achieved, for example, by means of a positive CaF2 lens and a negative LiF lens.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.